DMA disabled after lost interrupt (on ibook)

Hi people,
occasionally, I get "lost interrupt" messages in the logs of my ibook.
They typically show up when the hard disc drive is busy,
e.g. when lots of packages are extracted during "apt-get upgrade".
When this happens, the hard disc "stops" working for some
seconds and then continues to work.
The following messages are logged by the kernel, then:
kernel: ide-pmac lost interrupt, dma status: 8400
kernel: hda: lost interrupt
kernel: hda: dma_intr: status=0xd0 { Busy }
kernel:
kernel: ide: failed opcode was: unknown
kernel: hda: DMA disabled
kernel: ide0: reset: success
After issuing "hdparm -d1 /dev/hda", DMA is enabled again
and remains enabled, at least if the drive is not too
busy at that time. If all goes well, the kernel says:
kernel: hda: Enabling Ultra DMA 5
Some information on my ibook G4 (14' 933MHz model):
- My /proc/cpuinfo says:
processor : 0
cpu : 7455, altivec supported
clock : 707MHz
revision : 0.3 (pvr 8001 0303)
bogomips : 710.65
machine : PowerBook6,3
motherboard : PowerBook6,3 MacRISC3 Power Macintosh
detected as : 287 (iBook G4)
pmac flags : 0000001b
L2 cache : 256K unified
memory : 640MB
pmac-generation : NewWorld
Kernel version is 2.6.12.1 (but I noticed the "lost interrupt"
messages already with previous 2.6 versions).
If more information is necessary (Kernel config?), just tell me.
If this could be some HW problem, how can I test if this really
happens due to HW faults or because of a problem in the Linux kernel?
(Maybe I can figure this out with MacOSX? But how?)
Thanks for any help,
Vivenzio